Lec 25 Meiosis.docx

Meiosis
Lecture 25
Terminology
- Heredity- continuity of biological traits from one generation to the next:
o results from transmission of hereditary units (genes) from parents to offspring
- Variation- inherited differences among individuals of the same species
- Genetics- Scientific study of heredity and heredity variation
- DNA- type of nucleic acid that is a polymer of four different kinds of nucleotides
- Genes- units of hereditary information that are made of DNA and are located on chromosomes
- Chromosomes- organizational unit of heredity material in the nucleus of eukaryotic organisms
o Consist of a single long highly folded DNA molecules that are coiled along with proteins
o Contain genetic information arranged in a linear sequence and coded genes
- Locus- specific location on a chromosome that contains a gene
Comparison of asexual vs. Sexual reproduction
Asexual Sexual
Single individual is the sole parent Two parents give rise to offspring
Single parent passes on all its genes to its offspring Each parent passes on half its genes, to its
offspring
Offspring are genetically identical to the parent Offspring have an unique combination of genes
inherited from both parents
Results in a clone or genetically identical individual Results in greater genetic variation; offspring vary
genetically from their siblings and parents
Rarely, genetic differences occur as a result of
mutation, a change in DNA
A sexual reproduction of hydra
- In asexual reproduction- a single individual is the sole parent and passes copies of all its genes
on to its offspring
Definitions
- Sexual reproduction occurs only in eukaryotes
- During the formation of gametes (reproductive cells)
- The number of chromosomes is reduced by half (diploid)
- And returned to the full amount (diploid) when the two gametes fuse during fertilization
- Ploidy is a term referring to the number of sets of chromosomes - Haploid prganisms have one set of chromosomes; diploids have two
- Meiosis is a special type of nuclear division which segregates one copy of each homologous
chromosome into each new “gamete”
- Somatic (most) cells in the human body are produced by mitosis
- Germ line cells (gametes) are produced by meiosis
- The vast majority of cell divisions in the human body are mitotic, with meiosis being restricted to
the gonads
Life cycle
- In each generation, the doubling of chromosome number that results from fertilization is offset
by the halving of chromosome number that results from meiosis
- For humans, the number of chromosomes in haploid cell is 23 (n=23); the number of
chromosomes in the diploid zygote and all somatic cells arising from it is 46 (2n=46)
- Three sexual life cycles differing in the timing of meiosis and fertilization
- The common feature of all three cycles it eh alternation of these two key events, which
contribute to genetic variation among offspring.
Meiosis I
Prophase I
- Prophase I has unique event- the pairing of homologous chromosomes
- The process